Researchers use magnesium salt electrolyte additive to suppress growth of Li dendrites; more work needed to enhance Coulombic efficiency

Researchers from Shinshu University in Japan have developed a method to suppress the growth of lithium dendrites in Li-S and Li-air batteries. As reported in a paper in the RSC journal Physical Chemistry Chemical Physics, they used magnesium bis(trifluoromethanesulfonyl)amide [Mg(TFSA)2] as an electrolyteadditive to suppress the growth of Li dendrites, leveraging the occurrence of an alloying reaction between the initially substrate-deposited Mg and the subsequently deposited Li.

Lithium metal is a promising negative electrode material for advanced high energy density batteries due to its high theoretical capacity of 3860 mAh g−1 compared to that of the conventionally used graphite (LiC6: 372 mAh g−1). However, use of Li metal poses a number of safety risks, including the deposition of Li dendrites that penetrate the separator and induce internal short-circuiting. As a result, researchers have explored a number of different approaches to prevent dendrification, including the use of t3D matric substrates, electrolyte additives and the use of solid electrolytes.

Schematic illustration of proposed suppressing effect of Li dendritic growth by addition of Mg-salt.(a) In a commercially available electrolyte, Li deposition occurs inhomogeneously. By repeated deposition–dissolution cycling, the deposition morphology results in dendrite that causes a rapid capacity fading and thermal runway of batteries. (b) In an electrolyte containing Mg-salt, Mg-ion preferentially undergoes reduction to form metallic Mg on a substrate due to its higher standard electrode potential. Subsequently, Li deposition proceeds on the Mg and reacts with that to electrochemically form a binary Li–Mg alloy. Shimizu et al. Click to enlarge.

… our strategy relied on using an electrolyte containing a salt of a metal (e.g., Mg or Al) capable of electrochemical alloying with Li. In an electrolyte comprising both Li and Mg salts, Mg ions should be reduced in preference to Li ions and form a deposit of metallic Mg due to the higher electrode potential of Mg compared to that of Li (−2.36 vs. −3.04 V relative to the standard hydrogen electrode, respectively). Subsequently, Li deposition is expected to proceed on the previously deposited Mg to generate Li–Mg alloys.

Even in the case of initial excessive Li dendrite formation, the reduction of Mg ions estimated to occur in the region of high applied electric field should result in the formation of a Li−Mg alloy and thereby suppress further morphological changes. If the reaction occurs reversibly, Mg only dissolved and deposits between the electrolyte and the substrate, and Mg source should be not consumed even in long-term cycles, unlike SEI-forming electrolyte additives. The addition effect of alkaline-earth metal ion (Mg2+, Ca2+, Sr2+, Ba2+) to quaternary ammonium-based ionic liquid electrolytes on Li deposition morphology has already reported by other researchers, whereas we focused on addition into glyme-based organic electrolytes in the present study. Herein, we mainly examined the deposition and alloying behavior of Li in the presence of different concentrations of Mg(TFSA)2 under constant potential conditions.

—Shimizu et al.

Although the approach worked as a deterrent to dendrite formation, the researchers found it difficult to reverse, which is necessary in rechargable batteries. The researchers are studying the benefits of other types of magnesium salts, as well as working to improve the electrochemical stability of the salt combined with lithium to make reversal easier.

The researchers hope to solve the issues with this plating technology and eventually achieve a compact and high-capacity battery.

The use of a Mg salt-free electrolyte (1 M LiTFSA/G3) inevitably resulted in the growth of Li dendrites irrespective of the applied potential, with no improvements observed at a Mg(TFSA)2:LiTFSA molar ratio of 1:9. In contrast, Mg was deposited prior to Li in 0.5 M Mg(TFSA)2 + 0.5 M LiTFSA/G3, which resulted in electrochemical Li–Mg alloying. In the above system, the products formed at various applied potentials after 10 h corresponded to Li0.9Mg0.1 and exhibited a relatively smooth morphology. However, under constant current condition, although Li dendrite morphology was suppressed with increasing Mg-salt concentration in the electrolytes, their Coulombic efficiencies were not high. This is probably because the ion conduction was inhibited by the surface layer induced by the electrolyte decomposition and the passivation of Mg metal suppressed the dissolution. To enhance Coulombic efficiency, we are attempting to address the improvement of electrochemical stability and the optimization of electrolyte composition.

—Shimizu et al.

We aim to show the significantly improved reversibility of lithium deposition/dissolution and to realize stable operation for at least 1,000 cycles. The ultimate goal is to create batteries to run for 500 kilometers with full charge in electric vehicles.